How much manure from a dairy cow?

Happy dairy month!

As you all probably know June is dairy month and that means it’s time for a dairy manure story around here. As I got to thinking about what to discuss - I was working on a manure budget for the state of Iowa (how much manure to do we have, how much manure can we use, and how has this changed with time) and I started pondering about all the changes to agriculture and what it means for manure. Things like moving away from pasture and to confinement operations, adding storages, injecting manure when it is land applied all help us collect a larger fraction of the manure produced and better use it in our crop production systems. However, there is a deeper question, and that is, how has the amount of manure we got from a cow changed?

There are lots of things that could go into this, the production system we use, whether we scrape or flush manure, how much if anytime the cows spend on pasture or a loafing lot, but we are going to take a look at is how much manure the cow would excrete. Using the ASABE Manure Production standard (ASAE D384.2). To get some estimates of milk production the average annual milk production, in terms of pounds per had was obtained from the Survey of Iowa Agriculture for years 1924 through 2016. Over this time period milk production per cow has increased by almost 500% (figure 1); at the same time manure production per cow increased, but only by an estimated 40% (figure 2). This means that the we went getting about 0.1 pounds of milk for every pound of manure to more than 0.5 a pound of milk for every pound of manure, that’s a significant change.

Figure 1. average milk production per cow per year in Iowa.

Figure 2. Average manure production per cow per year in Iowa.

However, we can take a deeper a look at what some of these numbers mean for manure. For example, most of the weight of manure is water – but what about solids, nitrogen and phosphorus? Again we became more efficient here, with cows decreasing the amount of solids, nitrogen and phosphorus they excrete per pound of milk produced by 70-80%.  The actual excretion of solids, nitrogen, and phosphorus did increase, but only by 50% for solids, 16% for nitrogen, and 45% for phosphorus. If we do just a little more math and make some estimate about how much nitrogen is lost during storage, land application, and the availability of the nitrogen, we can make some estimates about how the manure’s nutrient content has changed (figure 3).

Figure 3. Estimated manure nutrient content.

While not entirely accurate as in the 1920’s through the 1960’s slurry manure systems were minimally used as animals were kept on pasture or raised in barns with bedding material and manure was handled as a solid. However, this work does provide some useful insight to understanding how this industry is changing, with animals that are much more efficient than they used to be, making more milk for every unit of manure we now manage. So next time you have a glass of refreshing milk, take a moment to ponder what the next 100 years will bring and what that could mean for how we manage manure.